New Technology is One Step Closer to Targeted Gene Therapy

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"A major challenge, however, has been creating the right "delivery vehicles" that can carry genes and molecules into the cells that need treatment, while avoiding the cells that do not.

Now, a team led by Caltech researchers has developed a gene-delivery system that can specifically target brain cells while avoiding the liver. This is important because a gene therapy intended to treat a disorder in the brain, for example, could also have the side effect of creating a toxic immune response in the liver, hence the desire to find delivery vehicles that only go to their intended target. The findings were shown in both mouse and marmoset models, an important step towards translating the technology into humans. ...

The key to this technology is the use of adeno-associated viruses, or AAVs, which have long been considered promising candidates for use as delivery vehicles. Over millions of years of evolution, viruses have evolved efficient ways to gain access into human cells, and for decades researchers have been developing methods to harness viruses' Trojan-Horse-like abilities for human benefit.

AAVs are made up of two major components: an outer shell, called a capsid, that is built from proteins; and the genetic material encased inside the capsid. To use recombinant AAVs for gene therapy, researchers remove the virus's genetic material from the capsid and replace it with the desired cargo, such as a particular gene or coding information for small therapeutic molecules.

"Recombinant AAVs are stripped of the ability to replicate, which leaves a powerful tool that is biologically designed to gain entrance into cells," ... 

The shape and composition of the capsid is a critical part of how the AAV enters into a cell. Researchers in the Gradinaru lab have been working for almost a decade on engineering AAV capsids that cross the blood-brain barrier (BBB) and to develop methods to select for and against certain traits, resulting in viral vectors more specific to certain cell types within the brain. ..."

From the abstract:

"Genetic intervention is increasingly being explored as a therapeutic option for debilitating disorders of the central nervous system. The safety and efficacy of gene therapies rely upon expressing a transgene in affected cells while minimizing off-target expression. Here we show organ-specific targeting of adeno-associated virus (AAV) capsids after intravenous delivery, which we achieved by employing a Cre-transgenic-based screening platform and sequential engineering of AAV-PHP.eB between the surface-exposed AA452 and AA460 of VP3. From this selection, we identified capsid variants that were enriched in the brain and targeted away from the liver in C57BL/6J mice. This tropism extends to marmoset (Callithrix jacchus), enabling robust, non-invasive gene delivery to the marmoset brain after intravenous administration. Notably, the capsids identified result in distinct transgene expression profiles within the brain, with one exhibiting high specificity to neurons. The ability to cross the blood–brain barrier with neuronal specificity in rodents and non-human primates enables new avenues for basic research and therapeutic possibilities unattainable with naturally occurring serotypes."

New Technology is One Step Closer to Targeted Gene Therapy | www.caltech.edu Broad and robust transgene expression across brain regions of marmoset is shown after systemic delivery with engineered capsid. Gene therapy is a powerful developing technology that has the potential to address myriad diseases. For example, Huntington's disease, a neurodegenerative disorder, is caused by a mutation in a single gene, and if researchers could go into specific cells and correct that defect, theoretically those cells could regain normal function.

AAV capsid variants with brain-wide transgene expression and decreased liver targeting after intravenous delivery in mouse and marmoset (open access)